Wet oxidation of phenol, cresols and nitrophenols catalyzed by activated carbon in acid and basic media

A commercial activated carbon, Industrial React FE01606A, was used as catalyst in the wet oxidation, in both acid and basic media, of phenolic pollutants, such as phenol, cresols and nitrophenols, currently found in industrial wastewaters. Reaction runs were carried out in a fixed-bed reactor (FBR) with concurrent upflow by feeding a 1000 mg L−1 aqueous solution on each pollutant concurrently with a gas oxygen flow. Temperature and oxygen pressure of the reactor were set to 160 °C and 16 bar, respectively. The basic medium was maintained by using 500 ppm sodium bicarbonate as buffer reagent to keep the pH in the range 7–8. The initial pH 3.5 was set by adding sulphuric acid. Oxidation intermediates were identified and their distribution with respect to the pollutant oxidation progress was measured. Utilizing these results, oxidation routes for each phenolic compound were deduced. The intermediates produced were diverse in acid and basic media and their composition explains the evolution of the corresponding toxicity measured at the reactor effluent. While under acidic conditions hydroxybenzoic acids, dihydroxyl benzenes and quinones were obtained as primary products, these last two compounds (more toxic than the original pollutant) were not detected in basic conditions, and with such media lower toxicities at the reactor exit were obtained. Moreover, the catalyst was found to be stable in the time range studied (300 h).